Method for the production of superabsorbers

ABSTRACT

A process for producing superabsorbents, comprising polymerization of a monomer solution and thermal surface postcrosslinking, wherein the monomer solution comprises at least 0.75% by weight of a hydroxyphosphonic acid or salts thereof, calculated on the basis of the total amount of monomer used, and at least 0.09% by weight of aluminum cations, calculated on the basis of the total amount of polymer particles used, is added to the polymer particles before, during or after the thermal surface postcrosslinking.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is U.S. national phase of International Application No.PCT/EP2017/069446, filed Aug. 1, 2017, which claims the benefit ofEuropean Application No. 16183579.8, filed Aug. 10, 2016.

The present invention relates to a process for producingsuperabsorbents, comprising polymerization of a monomer solution andthermal surface postcrosslinking, wherein the monomer solution comprisesat least 0.75% by weight of a hydroxyphosphonic acid, calculated on thebasis of the total amount of monomer used, and at least 0.09% by weightof aluminum cations, calculated on the basis of the total amount ofpolymer particles used, is added to the polymer particles before, duringor after the thermal surface postcrosslinking.

Superabsorbents are used to produce diapers, tampons, sanitary napkinsand other hygiene articles, but also as water-retaining agents in marketgardening. Superabsorbents are also referred to as water-absorbingpolymers.

The production of superabsorbents is described in the monograph “ModernSuperabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham,Wiley-VCH, 1998, pages 71 to 103.

The properties of the superabsorbents can be adjusted, for example, viathe amount of crosslinker used. With increasing amount of crosslinker,the centrifuge retention capacity (CRC) falls and the absorption under apressure of 21.0 g/cm² (AUL0.3 psi) passes through a maximum.

To improve the performance properties, for example gel bed permeability(GBP) and absorption under a pressure of 49.2 g/cm² (AUL0.7 psi),superabsorbent particles are generally surface postcrosslinked. Thisincreases the level of crosslinking of the particle surface, which canat least partly decouple the absorption under a pressure of 49.2 g/cm²(AUL0.7 psi) and the centrifuge retention capacity (CRC). This surfacepostcrosslinking can be performed in the aqueous gel phase. Preferably,however, dried, ground and sieved polymer particles (base polymer) aresurface coated with a surface postcrosslinker and thermally surfacepostcrosslinked. Crosslinkers suitable for that purpose are compoundswhich can form covalent bonds to at least two carboxylate groups of thepolymer particles.

WO 2011/113777 A1 describes the production of superabsorbents. Thisinvolves adding, during the polymerization or later, an inorganicphosphoric acid or salt thereof and an organic 2-hydroxy acid or saltthereof.

WO 2013/144026 A1 describes superabsorbents having surfaces that havebeen complexed with polyvalent metal ions and comprising a phosphonicacid derivative.

It was an object of the present invention to provide an improved processfor producing color-stable superabsorbents having high gel bedpermeability (GBP).

The object was achieved by a process for producing superabsorbents bypolymerizing a monomer solution or suspension comprising

-   a) at least one ethylenically unsaturated monomer which bears acid    groups and may have been at least partly neutralized,-   b) at least one crosslinker,-   c) at least one initiator,-   d) optionally one or more ethylenically unsaturated monomers    copolymerizable with the monomers mentioned under a) and-   e) optionally one or more water-soluble polymers,    comprising the steps of-   i) polymerizing the monomer solution to give a polymer gel,-   ii) optionally comminuting the resulting polymer gel,-   iii) drying the polymer gel,-   iv) grinding and classifying the dried polymer gel to give polymer    particles,-   v) thermally surface postcrosslinking the classified polymer    particles,-   vi) optionally reclassifying the surface postcrosslinked polymer    particles,-   vii) optionally recycling polymer particles removed in step iv)    upstream of step iii) and viii) optionally recycling polymer    particles removed in step vi) upstream of step iii),    which comprises adding to the monomer solution prior to step i) at    least 0.75% by weight of a hydroxyphosphonic acid or salts thereof,    calculated on the basis of the total amount of monomer a) used, and    adding to the polymer particles between step iv) and step vi) at    least 0.09% by weight of aluminum cations, calculated on the basis    of the total amount of polymer particles used.

Salts of hydroxyphosphonic acid used are preferably alkali metal salts,more preferably sodium and potassium salts, most preferably sodiumsalts. The neutralizable protons of the hydroxyphosphonic acid may beentirely or partly replaced by any desired alkali metal cations.

In a preferred embodiment of the present invention, the surfacepostcrosslinked polymer particles are reclassified in step vi) andpolymer particles removed in step vi) are recycled upstream of stepiii). The recycled polymer particles have a particle size of preferablyless than 250 μm, more preferably of less than 200 μm, most preferablyof less than 150 μm.

Preference is given to using 1-hydroxyethylidene-1,1′-diphosphonic acidas hydroxyphosphonic acid.

In a preferred embodiment of the present invention, aluminum cations areadded prior to step v).

Preferably, the aluminum cation is used in the form of aluminum sulfate.

Added to the monomer solution is preferably at least 0.80% by weight,more preferably at least 0.85% by weight, most preferably at least 0.90%by weight, of a hydroxyphosphonic acid or salts thereof, calculated onthe basis of the total amount of monomer a) used.

Added to the polymer particles between step iv) and step vi) ispreferably at least 0.10% by weight, more preferably at least 0.11% byweight, most preferably at least 0.12% by weight, of aluminum cations,calculated on the basis of the total amount of polymer particles used.

The present invention is based on the finding that a high colorstability and high gel bed permeability (GBP) can be achieved only witha relatively large amount of hydroxyphosphonic acid in the monomersolution and a relatively large amount of aluminum cations on theparticle surface. The use of sulfate as counterion leads to a distinctincrease in gel bed permeability (GBP).

In addition, it has been found that recycled surface postcrosslinkedsuperabsorbents do not absorb hydroxyphosphonic acid from the monomersolution. If such superabsorbents (“fines”) are recycled into theprocess, these polymer particles are not color-stable and lead to pointdiscolorations. Point discolorations of this kind are perceived as beingparticularly troublesome. Surprisingly, this is also observed when therecycled superabsorbents have been coated with a hydroxyphosphonic acidat the polymer particle stage. It is possible that some polymerparticles, especially the smaller polymer particles, are wetted onlyinadequately with the coating solution, if at all.

The production of the superabsorbents is described in detailhereinafter:

The superabsorbents are produced in step i) by polymerizing a monomersolution or suspension, and are typically water-insoluble.

The monomers a) are preferably water-soluble, i.e. their solubility inwater at 23° C. is typically at least 1 g/100 g of water, preferably atleast 5 g/100 g of water, more preferably at least 25 g/100 g of waterand most preferably at least 35 g/100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid and itaconicacid. Particularly preferred monomers are acrylic acid and methacrylicacid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturatedsulfonic acids, such as styrenesulfonic acid and2-acrylamido-2-methylpropanesulfonic acid (AMPS).

Impurities can have a considerable influence on the polymerization. Theraw materials used should therefore have a maximum purity. It istherefore often advantageous to specially purify the monomers a).Suitable purification processes are described, for example, in WO02/055469 A1, WO 03/078378 A1 and WO 2004/035514 A1. A suitable monomera) is, for example, an acrylic acid purified according to WO 2004/035514A1 and comprising 99.8460% by weight of acrylic acid, 0.0950% by weightof acetic acid, 0.0332% by weight of water, 0.0203% by weight ofpropionic acid, 0.0001% by weight of furfurals, 0.0001% by weight ofmaleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% byweight of hydroquinone monomethyl ether.

The proportion of acrylic acid and/or salts thereof in the total amountof monomers a) is preferably at least 50 mol %, more preferably at least90 mol %, most preferably at least 95 mol %.

The monomers a) typically comprise polymerization inhibitors, preferablyhydroquinone monoethers, as storage stabilizers.

The monomer solution comprises preferably up to 250 ppm by weight,preferably at most 130 ppm by weight, more preferably at most 70 ppm byweight, and preferably at least 10 ppm by weight, more preferably atleast 30 ppm by weight and especially around 50 ppm by weight, ofhydroquinone monoether, based in each case on the unneutralized monomera). For example, the monomer solution can be prepared by using anethylenically unsaturated monomer bearing acid groups with anappropriate content of hydroquinone monoether.

Preferred hydroquinone monoethers are hydroquinone monomethyl ether(MEHQ) and/or alpha-tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groupssuitable for crosslinking. Such groups are, for example, ethylenicallyunsaturated groups which can be polymerized free-radically into thepolymer chain, and functional groups which can form covalent bonds withthe acid groups of the monomer a). In addition, polyvalent metal saltswhich can form coordinate bonds with at least two acid groups of themonomer a) are also suitable as crosslinkers b).

Crosslinkers b) are preferably compounds having at least twopolymerizable groups which can be polymerized free-radically into thepolymer network. Suitable crosslinkers b) are, for example, ethyleneglycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycoldiacrylate, allyl methacrylate, trimethylolpropane triacrylate,triallylamine, tetraallylammonium chloride, tetraallyloxyethane, asdescribed in EP 0 530 438 A1, di- and triacrylates, as described in EP 0547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93/21237 A1, WO03/104299 A1, WO 03/104300 A1, WO 03/104301 A1 and DE 103 31 450 A1,mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE 103 31 456 A1 andDE 103 55 401 A1, or crosslinker mixtures, as described, for example, inDE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 02/032962 A2.

Preferred crosslinkers b) are pentaerythrityl triallyl ether,tetraallyloxyethane, methylenebismethacrylamide, 15-tuply ethoxylatedtrimethylolpropane triacrylate, polyethylene glycol diacrylate,trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethoxylatedand/or propoxylated glycerols which have been esterified with acrylicacid or methacrylic acid to give di- or triacrylates, as described, forexample, in WO 03/104301 A1. Di- and/or triacrylates of 3- to 10-tuplyethoxylated glycerol are particularly advantageous. Very particularpreference is given to di- or triacrylates of 1- to 5-tuply ethoxylatedand/or propoxylated glycerol. Most preferred are the triacrylates of 3-to 5-tuply ethoxylated and/or propoxylated glycerol, especially thetriacrylate of 3-tuply ethoxylated glycerol.

The amount of crosslinker b) is preferably 0.25% to 1.5% by weight, morepreferably 0.3% to 1.2% by weight and most preferably 0.4% to 0.8% byweight, calculated in each case on the basis of the total amount ofmonomer a) used. With rising crosslinker content, centrifuge retentioncapacity (CRC) falls and the absorption under a pressure of 21.0 g/cm²passes through a maximum.

Initiators c) used may be all compounds which generate free radicalsunder the polymerization conditions, for example thermal initiators,redox initiators or photoinitiators. Suitable redox initiators aresodium peroxodisulfate/ascorbic acid, hydrogen peroxide/ascorbic acid,sodium peroxodisulfate/sodium bisulfite and hydrogen peroxide/sodiumbisulfite. Preference is given to using mixtures of thermal initiatorsand redox initiators, such as sodium peroxodisulfate/hydrogenperoxide/ascorbic acid. The reducing component used is preferably thedisodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of thesodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite. Such mixtures areobtainable as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals;Heilbronn; Germany).

Ethylenically unsaturated monomers d) copolymerizable with theethylenically unsaturated monomers a) bearing acid groups are, forexample, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethylmethacrylate, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate.

The water-soluble polymers e) used may be polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, modified cellulose,such as methyl cellulose or hydroxyethyl cellulose, gelatin, polyglycolsor polyacrylic acids, preferably starch, starch derivatives and modifiedcellulose.

Typically, an aqueous monomer solution is used. The water content of themonomer solution is preferably from 40% to 75% by weight, morepreferably from 45% to 70% by weight and most preferably from 50% to 65%by weight. It is also possible to use monomer suspensions, i.e. monomersolutions with solubility-exceeding monomer a), for example sodiumacrylate. As the water content rises, the energy expenditure in thesubsequent drying rises and, as the water content falls, the heat ofpolymerization can only be removed inadequately.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. The monomer solution can therefore be freed ofdissolved oxygen before the polymerization by inertization, i.e. flowingan inert gas through, preferably nitrogen or carbon dioxide. The oxygencontent of the monomer solution is preferably lowered before thepolymerization to less than 1 ppm by weight, more preferably to lessthan 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

Suitable reactors for the polymerization in step i) are, for example,kneading reactors or belt reactors. In the kneader, the polymer gelformed in the polymerization of an aqueous monomer solution orsuspension is comminuted continuously by, for example, contrarotatorystirrer shafts, as described in WO 2001/038402 A1. Polymerization on abelt is described, for example, in DE 38 25 366 A1 and U.S. Pat. No.6,241,928. Polymerization in a belt reactor forms a polymer gel whichhas to be comminuted in a further step, step ii), for example in anextruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained bymeans of a kneader can additionally be extruded in step ii).

The acid groups of the resulting polymer gels have typically been partlyneutralized. Neutralization is preferably carried out at the monomerstage. This is typically accomplished by mixing in the neutralizingagent as an aqueous solution or else preferably as a solid. The degreeof neutralization is preferably from 25 to 85 mol %, more preferablyfrom 30 to 80 mol % and most preferably from 40 to 75 mol %, for whichthe customary neutralizing agents can be used, preferably alkali metalhydroxides, alkali metal oxides, alkali metal carbonates or alkali metalhydrogencarbonates and also mixtures thereof. Instead of alkali metalsalts, it is also possible to use ammonium salts. Particularly preferredalkali metals are sodium and potassium, but very particular preferenceis given to sodium hydroxide, sodium carbonate or sodiumhydrogencarbonate and also mixtures thereof. Solid carbonates andhydrogencarbonates can also be introduced here in encapsulated form,preferably into the monomer solution directly prior to thepolymerization, into the polymer gel during or after the polymerizationand prior to the drying thereof. The encapsulation is effected bycoating of the surface with an insoluble or only gradually solublematerial (for example by means of film-forming polymers, of inertinorganic materials or of fusible organic materials) which delays thedissolution and reaction of the solid carbonate or hydrogencarbonate tosuch a degree that carbon dioxide is not released until during thedrying and the superabsorbent formed has high internal porosity.

Optionally, a surfactant can be added to the monomer solution before orduring the polymerization and the monomer solution can then be foamedbefore or during the polymerization with an inert gas or water vapor orby vigorous stirring. The surfactant may be anionic, cationic,zwitterionic or else nonionic. Preference is given to using askin-friendly surfactant.

The polymer gel is then dried with an air circulation belt drier in stepiii) until the residual moisture content is preferably 0.5 to 10% byweight, more preferably 1 to 6% by weight and most preferably 1.5 to 4%by weight, the residual moisture content being determined by EDANArecommended test method No. WSP 230.2-05 “Mass Loss Upon Heating”. Inthe case of too high a residual moisture content, the dried polymer gelhas too low a glass transition temperature T_(g) and can be processedfurther only with difficulty. In the case of too low a residual moisturecontent, the dried polymer gel is too brittle and, in the subsequentcomminution steps, undesirably large amounts of polymer particles withan excessively low particle size are obtained (“fines”). The solidscontent of the polymer gel before the drying is preferably from 25% to90% by weight, more preferably from 35% to 70% by weight, mostpreferably from 40% to 60% by weight. Subsequently, the dried polymergel is crushed and optionally coarsely comminuted.

Thereafter, the dried polymer gel is ground and classified in step iv),and the apparatus used for grinding may typically be single ormultistage roll mills, preferably two- or three-stage roll mills, pinmills, hammer mills or vibratory mills.

The average particle size of the polymer particles removed as theproduct fraction in step xii) is preferably from 150 to 850 μm, morepreferably from 250 to 600 μm, very particularly from 300 to 500 μm. Theaverage particle size of the product fraction may be determined by meansof EDANA recommended test method No. WSP 220.2-05 “Particle SizeDistribution”, where the proportions by mass of the screen fractions areplotted in cumulated form and the average particle size is determinedgraphically. The average particle size here is the value of the meshsize which arises for a cumulative 50% by weight.

The proportion of polymer particles having a particle size of greaterthan 150 μm is preferably at least 90% by weight, more preferably atleast 95% by weight, most preferably at least 98% by weight.

Polymer particles with too small a particle size lower the gel bedpermeability (GBP). The proportion of excessively small polymerparticles (“fines”) should therefore be small.

Excessively small polymer particles are therefore typically removed andrecycled into the process, preferably before, during or immediatelyafter the polymerization in step i), i.e. prior to the drying of thepolymer gel in step iii). The excessively small polymer particles can bemoistened with water and/or aqueous surfactant before or during therecycling.

It is also possible to remove excessively small polymer particles inlater process steps, for example after the surface postcrosslinking instep v) or another coating step. In this case, the excessively smallpolymer particles recycled are surface postcrosslinked or coated inanother way, for example with fumed silica.

If a kneading reactor is used for polymerization, the excessively smallpolymer particles are preferably added in step i) during the last thirdof the polymerization. However, it is also possible to incorporate theexcessively small polymer particles into the polymer gel in a step ii)downstream of the polymerization reactor, for example in a kneader orextruder.

If the excessively small polymer particles are added at a very earlystage, for example actually to the monomer solution, this lowers thecentrifuge retention capacity (CRC) of the resulting polymer particles.However, this can be compensated, for example, by adjusting the amountof crosslinker b) used.

The proportion of polymer particles having a particle size of at most850 μm is preferably at least 90% by weight, more preferably at least95% by weight, most preferably at least 98% by weight.

The proportion of polymer particles having a particle size of at most600 μm is preferably at least 90% by weight, more preferably at least95% by weight, most preferably at least 98% by weight.

Polymer particles of excessively large particle size lower the freeswell rate. The proportion of excessively large polymer particles shouldtherefore likewise be low. Excessively large polymer particles aretherefore typically removed and recycled into the grinding.

To further improve the properties, the polymer particles can bethermally surface postcrosslinked in step v). Suitable surfacepostcrosslinkers are compounds which comprise groups which can formcovalent bonds with at least two carboxylate groups of the polymerparticles.

Suitable compounds are, for example, polyfunctional amines,polyfunctional amido amines, polyfunctional epoxides, as described in EP0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctionalalcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450922 A2, β-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S.Pat. No. 6,239,230.

Additionally described as suitable surface postcrosslinkers are cycliccarbonates in DE 40 20 780 C1, 2-oxazolidinone and derivatives thereof,such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1, bis- andpoly-2-oxazolidinones in DE 198 07 992 C1, 2-oxotetrahydro-1,3-oxazineand derivatives thereof in DE 198 54 573 A1, N-acyl-2-oxazolidinones inDE 198 54 574 A1, cyclic ureas in DE 102 04 937 A1, bicyclic amidoacetals in DE 103 34 584 A1, oxetanes and cyclic ureas in EP 1 199 327A2 and morpholine-2,3-dione and derivatives thereof in WO 03/031482 A1.

Preferred surface postcrosslinkers are ethylene carbonate, ethyleneglycol diglycidyl ether, reaction products of polyamides withepichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.

In addition, it is also possible to use surface postcrosslinkers whichcomprise additional polymerizable ethylenically unsaturated groups, asdescribed in DE 37 13 601 A1.

The amount of surface postcrosslinker is preferably 0.001% to 3% byweight, more preferably 0.02% to 1% by weight and most preferably 0.05%to 0.2% by weight, based in each case on the polymer particles.

The surface postcrosslinking is typically performed in such a way that asolution of the surface postcrosslinker is sprayed onto the driedpolymer particles. After the spray application, the polymer particlescoated with surface postcrosslinker are surface postcrosslinked anddried, and the surface postcrosslinking reaction can take place bothbefore and during the drying.

The spray application of a solution of the surface postcrosslinker ispreferably performed in mixers with moving mixing tools, such as screwmixers, disk mixers and paddle mixers. Particular preference is given tohorizontal mixers such as paddle mixers, very particular preference tovertical mixers. The distinction between horizontal mixers and verticalmixers is made by the position of the mixing shaft, i.e. horizontalmixers have a horizontally mounted mixing shaft and vertical mixers avertically mounted mixing shaft. Suitable mixers are, for example,horizontal Pflugschar® plowshare mixers (Gebr. Lödige Maschinenbau GmbH;Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV;Doetinchem; the Netherlands), Processall Mixmill mixers (ProcessallIncorporated; Cincinnati; USA) and Schugi Flexomix® (Hosokawa Micron BV;Doetinchem; the Netherlands). However, it is also possible to spray onthe surface postcrosslinker solution in a fluidized bed.

The surface postcrosslinkers are typically used in the form of anaqueous solution. The penetration depth of the surface postcrosslinkerinto the polymer particles can be adjusted via the content of nonaqueoussolvent and total amount of solvent.

When exclusively water is used as the solvent, a surfactant isadvantageously added. This improves the wetting characteristics andreduces the tendency to form lumps. However, preference is given tousing solvent mixtures, for example isopropanol/water,1,3-propanediol/water and propylene glycol/water, where the mixing ratioin terms of mass is preferably from 20:80 to 40:60.

The surface postcrosslinking is preferably performed in contact driers,more preferably shovel driers, most preferably disk driers. Suitabledriers are, for example, Hosokawa Bepex® Horizontal Paddle Dryer(Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Bepex® Disc Dryer(Hosokawa Micron GmbH; Leingarten; Germany), Holo-Flite® driers (MetsoMinerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARAMachinery Europe; Frechen; Germany). Moreover, fluidized bed driers mayalso be used.

The surface postcrosslinking can be effected in the mixer itself, byheating the jacket or blowing in warm air. Equally suitable is adownstream drier, for example a shelf drier, a rotary tube oven or aheatable screw. It is particularly advantageous to effect mixing andthermal surface postcrosslinking in a fluidized bed drier.

Preferred reaction temperatures are in the range of 100 to 250° C.,preferably 120 to 220° C., more preferably 130 to 210° C., mostpreferably 150 to 200° C. The preferred dwell time at this temperatureis preferably at least 10 minutes, more preferably at least 20 minutes,most preferably at least 30 minutes, and typically at most 60 minutes.

In a preferred embodiment of the present invention, the polymerparticles are cooled after the surface postcrosslinking. The cooling ispreferably performed in contact coolers, more preferably paddle coolersand most preferably disk coolers. Suitable coolers are, for example,Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH;Leingarten; Germany), Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH;Leingarten; Germany), Holo-Flite® coolers (Metso Minerals IndustriesInc.; Danville; USA) and Nara Paddle Cooler (NARA Machinery Europe;Frechen; Germany). Moreover, fluidized bed coolers may also be used.

In the cooler, the polymer particles are cooled to preferably 40 to 90°C., more preferably 45 to 80° C., most preferably 50 to 70° C.

Subsequently, the surface postcrosslinked polymer particles can beclassified again, with excessively small and/or excessively largepolymer particles being removed and recycled into the process.

To further improve the properties, the surface postcrosslinked polymerparticles can be coated or remoisturized.

The remoisturizing is preferably performed at 40 to 120° C., morepreferably at 50 to 110° C., most preferably at 60 to 100° C. Atexcessively low temperatures the polymer particles tend to form lumps,and at higher temperatures water already evaporates to a noticeabledegree. The amount of water used for remoisturizing is preferably from1% to 10% by weight, more preferably from 2% to 8% by weight and mostpreferably from 3% to 5% by weight. The remoisturizing increases themechanical stability of the polymer particles and reduces their tendencyto static charging. The remoisturizing is advantageously performed inthe cooler after the thermal surface postcrosslinking.

Suitable coatings for improving the free swell rate and the gel bedpermeability (GBP) are, for example, inorganic inert substances, such aswater-insoluble metal salts, organic polymers, cationic polymers and di-or polyvalent metal cations. Suitable coatings for dust binding are, forexample, polyols. Suitable coatings for counteracting the undesiredcaking tendency of the polymer particles are, for example, fumed silica,such as Aerosil® 200, and surfactants, such as Span® 20. Suitablecoatings for dust binding, for reducing the tendency to caking and forincreasing the mechanical stability are polymer dispersions as describedin EP 0 703 265 B1, and waxes as described in U.S. Pat. No. 5,840,321.

The present invention further provides the superabsorbents produced bythe process of the invention, where the superabsorbent particles have acentrifuge retention capacity of at least 25 g/g, an absorption under apressure of 49.2 g/cm² of at least 15 g/g, a gel bed permeability of atleast 60 darcies, and the superabsorbent particles, after storage at atemperature of 70° C. and a relative humidity of 80% for 14 days, havean L value of at least 67, an a value of less than 7.0 and a b value ofless than 14.5.

The superabsorbent particles of the invention have high color stabilityeven in a mixture with cellulose as typically exists in diapers. Bycontrast, superabsorbent particles in which the color stabilizer hasmerely been applied to the surface thereof have distinctly lower colorstability when mixed with cellulose.

The superabsorbent particles produced by the process of the inventionhave a centrifuge retention capacity (CRC) of preferably at least 26g/g, more preferably at least 27 g/g, most preferably at least 28 g/g.The centrifuge retention capacity (CRC) of the superabsorbent particlesis typically less than 60 g/g.

The superabsorbent particles produced by the process of the inventionhave an absorption under a pressure of 49.2 g/cm² (AUL0.7 psi) ofpreferably at least 16 g/g, more preferably at least 17 g/g, mostpreferably at least 18 g/g. The absorption under a pressure of 49.2g/cm² (AUL0.7 psi) of the superabsorbent particles is typically lessthan 35 g/g.

The superabsorbent particles produced by the process of the inventionhave a gel bed permeability (GBP) of preferably at least 65 darcies,more preferably at least 70 darcies, most preferably at least 75darcies. The gel bed permeability (GBP) of the superabsorbent particlesis typically less than 400 darcies.

The superabsorbent particles produced by the process of the invention,after storage at a temperature of 70° C. and a relative humidity of 80%for 14 days, have an L value of preferably at least 68, more preferablyat least 69, most preferably at least 70.

The superabsorbent particles produced by the process of the invention,after storage at a temperature of 70° C. and a relative humidity of 80%for 14 days, preferably have an a value of less than 6.6 and a b valueof less than 14.0, more preferably an a value of less than 6.2 and a bvalue of less than 13.5, most preferably an a value of less than 5.8 anda b value of less than 13.0.

Methods:

The standard test methods described hereinafter and designated “WSP” aredescribed in: “Standard Test Methods for the Nonwovens Industry”, 2005edition, published jointly by the Worldwide Strategic Partners EDANA(Avenue Eugène Plasky, 157, 1030 Brussels, Belgium, www.edana.org) andINDA (1100 Crescent Green, Suite 115, Cary, N.C. 27518, USA,www.inda.org). This publication is obtainable both from EDANA and fromINDA.

The measurements should, unless stated otherwise, be conducted at anambient temperature of 23±2° C. and a relative air humidity of 50±10%.The water-absorbing polymer particles are mixed thoroughly before themeasurement.

Centrifuge Retention Capacity

The centrifuge retention capacity (CRC) is determined by EDANArecommended test method No. WSP 241.2-05 “Fluid Retention Capacity inSaline, After Centrifugation”.

Absorption Under a Pressure of 21.0 g/Cm² (Absorption Under Load)

The absorption under a pressure of 21.0 g/cm² (AUL0.3 psi) of thewater-absorbing polymer particles is determined by EDANA recommendedtest method No. WSP 242.2-05 “Absorption Under Pressure, GravimetricDetermination”.

Absorption Under a Pressure of 49.2 g/Cm² (Absorption Under Load)

The absorption under a pressure of 49.2 g/cm² (AUL0.7 psi) is determinedanalogously to EDANA recommended test method No. WSP 242.2-05“Absorption Under Pressure, Gravimetric Determination”, except that apressure of 49.2 g/cm² (AUL0.7 psi) is established rather than apressure of 21.0 g/cm² (AUL0.3 psi).

Gel Bed Permeability

The gel bed permeability (GBP) of a swollen gel layer under a pressureof 0.3 psi (2070 Pa) is, as described in US 2005/0256757 (paragraphs[0061] and [0075]), determined as the gel bed permeability of a swollengel layer of water-absorbing polymer particles.

CIE Color Number (L, a, b)

The color analysis is carried out according to the CIELAB method(Hunterlab, volume 8, 1996, book 7, pages 1 to 4) with a “LabScan XE S/NLX17309” colorimeter (HunterLab, Reston, US). This method describes thecolors via the coordinates L, a and b of a three-dimensional system. Lindicates the brightness, where L=0 means black and L=100 white. Thevalues of a and b indicate the positions of the color on the red/greenand yellow/blue color axes respectively, where +a represents red, −arepresents green, +b represents yellow and −b represents blue. The HC60is calculated by the formula HC60=L−3b.

The color measurement corresponds to the three-area method according toDIN 5033-6.

EXAMPLES Example 1

A monomer solution which is composed of 406.8 g of acrylic acid, 4271.4g of aqueous sodium acrylate solution (37.3% strength by weight), 130.4g of water and 5.86 g of 3-tuply ethoxylated glycerol triacrylate(purity about 85% by weight) and has been freed of atmospheric oxygenwith nitrogen gas for 30 minutes was polymerized in an LUK 8.0 K2polymerization reactor having two axially parallel shafts (CoperionWerner & Pfleiderer GmbH & Co. KG; Stuttgart, Germany). Additionallyadded to the monomer solution were 77.29 g of an aqueous solution of thedisodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (20% strengthby weight). The polymerization was initiated by adding 15.84 g ofaqueous sodium peroxodisulfate solution (15% strength by weight), 1.41 gof aqueous hydrogen peroxide (1% strength by weight) and 91.12 g ofaqueous ascorbic acid solution (0.5% strength by weight). Thepolymerization had ended after about 30 minutes. The resulting polymergel was ground three times with the aid of a commercial meat grinderwith a 6 mm die plate, and dried in a laboratory drying cabinet at 175°C. for 90 minutes. The dried polymer was ground and sieved to a particlesize of 150 to 710 μm. The base polymer thus produced had a centrifugeretention capacity (CRC) of 38 g/g.

For surface postcrosslinking, 1000 g of base polymer were coated in anM5R plowshare mixer (Gebrüder Lödige Maschinenbau GmbH; Paderborn;Germany) at 23° C. and a shaft speed of 200 revolutions per minute bymeans of two two-phase spray nozzles with the following solutions:

Solution I: 0.50 g of N-(2-hydroxyethyl)-2-oxazolidinone

-   -   0.50 g of propane-1,3-diol    -   33.10 g of aqueous aluminum sulfate solution (26.8% strength by        weight)

Solution II: 10.0 g of isopropanol

After the spray application of the two solutions, the temperature wasincreased to 195° C. and the reaction mixture was held at thistemperature and a shaft speed of 60 revolutions per minute for 75minutes. Subsequently, the reaction mixture was resieved at 23° C. to aparticle size of 150 to 710 μm.

The superabsorbent particles obtained were analyzed. To determine colorstability, the superabsorbent particles were stored at 70° C. and arelative humidity of 80% for 14 days. The results are compiled in table1.

Example 2 (Comparative Example)

The procedure was as in example 1. Rather than 77.29 g of an aqueoussolution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonicacid (20% strength by weight), only 36.61 g of an aqueous solution ofthe disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (20%strength by weight) were added to the monomer solution.

The superabsorbent particles obtained were analyzed. To determine colorstability, the superabsorbent particles were stored at 70° C. and arelative humidity of 80% for 14 days. The results are compiled in table1.

Example 3 (Comparative Example)

The procedure was as in example 1. Surface postcrosslinking wasaccomplished using, rather than 42.90 g of aqueous aluminum sulfatesolution (26.8% strength by weight), only 11.10 g of aqueous aluminumsulfate solution (26.8% strength by weight).

The superabsorbent particles obtained were analyzed. To determine colorstability, the superabsorbent particles were stored at 70° C. and arelative humidity of 80% for 14 days. The results are compiled in table1.

Example 4 (Comparative Example)

The procedure was as in example 1. The monomer solution did not compriseany disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid. Instead,15.46 g of the disodium salt of 2-hydroxy-2-sulfonatoacetic acid wereadded to the monomer solution.

During the polymerization, the polymer gel formed wound around theshafts of the polymerization reactor. The experiment had to be stopped.

Example 5 (Comparative Example)

The procedure was as in example 1. The monomer solution did not compriseany disodium salt of 1-hydroxyethylidene-1,1-diphosphonic acid.

The superabsorbent particles obtained were analyzed. To determine colorstability, the superabsorbent particles were stored at 70° C. and arelative humidity of 80% for 14 days. The results are compiled in table1.

Example 6

The procedure was as in example 1. No aluminum sulfate was used forsurface postcrosslinking. Instead, 80.8 g of aqueous aluminum trilactatesolution (18.9% strength by weight) were used.

The superabsorbent particles obtained were analyzed. To determine colorstability, the superabsorbent particles were stored at 70° C. and arelative humidity of 80% for 14 days. The results are compiled in table1.

TABLE 1 CRC AUL0.7 psi GBP Ex. Monomer solution SXL [g/g] [g/g][darcies] L a b 1 9500 ppm Cublen 1500 ppm Al³⁺ 29.8 21.4 109 74 5.3 12(as sulfate) 2*) 4500 ppm Cublen 1500 ppm Al³⁺ 30.4 21.5 101 63 7.3 15(as sulfate) 3*) 9500 ppm Cublen  500 ppm Al³⁺ 31.1 23.3 26 85 0.9 8.6(as sulfate) 4*) 9500 ppm Blancolen **) — — — — — — 5*) No Cublen 1500ppm Al³⁺ 29.9 20.6 118 50 9.1 16.7 (as sulfate) 6*) 9500 ppm Cublen 1500ppm Al³⁺ 29.3 25.3 17 79 3.7 11 (as lactate) SXL: surfacepostcrosslinking Cublen: disodium salt of1-hydroxyethylidene-1,1-diphosphonic acid Blancolen: disodium salt of2-hydroxy-2-sulfonatoacetic acid *)comparative experiment **) notpossible, polymerization stopped

The results of examples 1 to 3 show that superabsorbents having highcolor stability and high gel bed permeability (GBP) are obtained onlywith a large amount of hydroxyphosphonic acid (Cublen) and a largeamount of aluminum cations.

Example 4 shows that 2-hydroxysulfonic acids (Blancolen), in the amountsin the monomer solution that are necessary for color stabilization,interfere in the polymerization.

Examples 1 and 6 show the advantages of sulfate over lactate ascounterion.

Example 7

By mixing water, aqueous sodium hydroxide solution and acrylic acid, a43.0% strength by weight acrylic acid/sodium acrylate solution wasprepared. The degree of neutralization of the monomer solution thusprepared was 72.0 mol %.

The monomer solution was degassed with nitrogen at about 23° C. Thepolyethylenically unsaturated crosslinker used was 3-tuply ethoxylatedglyceryl triacrylate (purity about 85% by weight). The use amount of3-tuply ethoxylated glycerol triacrylate, calculated on the basis of theamount of acrylic acid used, was 0.16% by weight.

Additionally added to the monomer solution was a 20% by weight aqueoussolution of the disodium salt of 1-hydroxyethylidene-1,1-diphosphonicacid. The use amount of disodium salt of1-hydroxyethylidene-1,1-diphosphonic acid, calculated on the basis ofthe amount of acrylic acid used, was 0.30% by weight.

To initiate the free-radical polymerization, the following componentswere used: hydrogen peroxide (0.002% by weight of a 1.0% by weightaqueous solution), sodium peroxodisulfate (0.15% by weight of a 15% byweight aqueous solution), and ascorbic acid (0.01% by weight of a 0.5%by weight aqueous solution). The percentages by weight were calculatedbased on the total amount of acrylic acid used.

The individual components were metered continuously into a List ORP 10kneader reactor (List AG, Arisdorf, Switzerland). The throughput of themonomer solution was 40 kg/h.

The reaction solution had a feed temperature of about 23° C. The dwelltime of the reaction mixture in the reactor was about 15 minutes. Afterpolymerization and gel comminution, 1 kg of aqueous polymer gel in eachcase was applied to drying sheets with a base of 300 μm sieve mesh anddried in an air circulation drying cabinet at 175° C. for 60 minutes.The dried polymer gel was ground and classified by means of a roll mill.

To determine color stability, the superabsorbent particles were storedat 70° C. and a relative humidity of 80% for 14 days. The results arecompiled in table 2.

Example 8

The procedure was as in example 7. In the first third of the reactor,0.86 kg/h of superabsorbent with a particle size of less than 150 μm wasadditionally added. The added superabsorbent particles had beenthermally surface postcrosslinked and did not comprise any disodium saltof 1-hydroxyethylidene-1,1-diphosphonic acid.

To determine color stability, the superabsorbent particles were storedat 70° C. and a relative humidity of 80% for 14 days. The results arecompiled in table 2.

Example 9

The procedure was as in example 7. In the first third of the reactor,1.72 kg/h of superabsorbent with a particle size of less than 150 μmwere additionally added. The added superabsorbent particles had beenthermally surface postcrosslinked and did not comprise any disodium saltof 1-hydroxyethylidene-1,1-diphosphonic acid.

To determine color stability, the superabsorbent particles were storedat 70° C. and a relative humidity of 80% for 14 days. The results arecompiled in table 2.

Example 10

The procedure was as in example 7. In the first third of the reactor,2.58 kg/h of superabsorbent with a particle size of less than 150 μmwere additionally added. The added superabsorbent particles had beenthermally surface postcrosslinked and did not comprise any disodium saltof 1-hydroxyethylidene-1,1-diphosphonic acid.

To determine color stability, the superabsorbent particles were storedat 70° C. and a relative humidity of 80% for 14 days. The results arecompiled in table 2.

Example 11

The procedure was as in example 7. In the first third of the reactor,3.44 kg/h of superabsorbent with a particle size of less than 150 μmwere additionally added. The added superabsorbent particles had beenthermally surface postcrosslinked and did not comprise any disodium saltof 1-hydroxyethylidene-1,1-diphosphonic acid.

To determine color stability, the superabsorbent particles were storedat 70° C. and a relative humidity of 80% for 14 days. The results arecompiled in table 2.

TABLE 2 Superabsorbent added Ex. (calculated based on polymer produced)Discolorations 7 none + 8  5% by weight − 9 10% by weight − 10 15% byweight −− 11 20% by weight −−− No visible point discolorations + Fewvisible point discolorations − Distinct, visible point discolorations −−Very distinct, visible point discolorations −−−

The results of examples 7 to 11 show that the thermally surfacepostcrosslinked superabsorbent particles no longer absorb any disodiumsalt of 1-hydroxyethylidene-1,1-diphosphonic acid and do not becomediscolored during storage. The point discolorations increase with theamount of added superabsorbent.

The invention claimed is:
 1. A process for producing superabsorbentparticles by polymerizing a monomer solution or suspension comprising a)at least one ethylenically unsaturated monomer which bears an acid groupand may have been at least partly neutralized, b) at least onecrosslinker, c) at least one initiator, d) optionally one or moreethylenically unsaturated monomer copolymerizable with the monomermentioned under a) and e) optionally one or more water-soluble polymer,comprising i) polymerizing the monomer solution to give a polymer gel,ii) optionally comminuting the resulting polymer gel, iii) drying thepolymer gel, iv) grinding and classifying the dried polymer gel to givepolymer particles, v) thermally surface postcrosslinking the classifiedpolymer particles, vi) optionally reclassifying the surfacepostcrosslinked polymer particles, vii) optionally recycling polymerparticles removed in step iv) upstream of step iii) and viii) optionallyrecycling polymer particles removed in step vi) upstream of step iii),which comprises adding to the monomer solution prior to step i) at least0.75% by weight of a hydroxyphosphonic acid or salts thereof, calculatedon the basis of the total amount of monomer a) used, and adding to thepolymer particles between step iv) and step vi) at least 0.09% by weightof aluminum cations, calculated on the basis of the total amount ofpolymer particles used.
 2. The process according to claim 1, wherein thepolymer particles removed in step vi) and recycled upstream of step iii)have a particle size of less than 150 μm.
 3. The process according toclaim 1, wherein 1-hydroxyethylidene-1,1′-diphosphonic acid is used asthe hydroxyphosphonic acid.
 4. The process according to claim 1, whereinthe aluminum cations are added prior to step v).
 5. The processaccording to claim 1, wherein the aluminum cation is used in the form ofaluminum sulfate.
 6. The process according to claim 1, wherein at least0.8% by weight of a hydroxyphosphonic acid or salts thereof, calculatedon the basis of the total amount of monomer a) used, is added to themonomer solution prior to step i).
 7. The process according to claim 1,wherein at least 0.85% by weight of a hydroxyphosphonic acid or saltsthereof, calculated on the basis of the total amount of monomer a) used,is added to the monomer solution prior to step i).
 8. The processaccording to claim 1, wherein at least 0.1% by weight of aluminumcations, calculated on the basis of the total amount of polymerparticles used, is added to the polymer particles after step iv).
 9. Theprocess according to claim 1, wherein at least 0.11% by weight ofaluminum cations, calculated on the basis of the total amount of polymerparticles used, is added to the polymer particles after step iv).